This invention relates to a thyristor having a low on-state voltage.
Thyristors, as disclosed in U.S. Pat. No. 3,422,323 and U.S. Pat. No. Re. 27,440 for example, comprise generally a four-layer semiconductor substrate of PNPN structure consisting of continuously arranged layers of alternately different conductivity types forming a PN junction between each of the adjacent layers, a pair of main electrodes making ohmic contact with the outer P-type and N-type layers respectively of the semiconductor substrate, and trigger means for switching over the regions between the main electrodes from off-state to on-state. The operating function of the thyristor of this kind is such that the thyristor remains in the off-state so long as a forward voltage lower than a critical voltage called a forward break-over voltage is kept applied across the main electrodes, and switch-over from the off-state to the on-state takes place in response to the application of a trigger signal from the trigger means.
In the thyristor of the kind described above having such a switching function, the thyristor performance can be improved by decreasing the impedance of the regions between the main electrodes in the on-state, that is, by reducing the on-state voltage. Principal factors which determine this on-state voltage of the thyristor include the sum of the voltage across the PN junctions and the voltage drop in the intermediate P-type and N-type layers due to the resistance thereof. This voltage drop in the intermediate P-type and N-type layers due to the resistance thereof become less when the amount of carriers injected into the intermediate layers from the outer layers is greater, and the amount of carriers injected into the intermediate layers increases with the increase in the amount of impurities in the outer layers. Therefore, common practice has been such that the amount of impurities in the outer layers is sufficiently increased to reduce the voltage drop in the intermediate layers thereby reducing the on-state voltage of the thyristor. However, this increase in the amount of impurities in the outer layers gives rise to a reverse effect of corresponding increase in the voltage applied across the PN junctions. There has therefore been a limit to the desired reduction of the on-state voltage of the thyristor by the increase in the amount of impurities doped in the outer layers of the thyristor.